Methods of directional drilling and forming kickoff plugs using self-degrading cement in subterranean well bores

ABSTRACT

Methods of drilling a well bore in a subterranean formation comprising placing a self-degrading kickoff plug or the like in the well bore are provided. An example of a method is a method comprising: providing a self-degrading cement composition that comprises a degradable material, an acid source, a base source, and a water source; placing the self-degrading cement composition in a desired location in a well bore that penetrates a subterranean formation; and allowing the self-degrading cement composition to set to form a hardened kickoff plug.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. Nos. 11/048,272 and 11/048,591, both entitled “Self-DegradingCement Compositions and Methods of Using Self-Degrading CementCompositions in Subterranean Formations,” both filed on Feb. 1, 2005,the entirety of which are herein incorporated by reference, and fromwhich priority is claimed pursuant to 35 U.S.C. §120. This applicationis also related to co-pending U.S. patent application Ser. No. ______,Attorney Docket Number HES 2005-IP-016968U1, entitled “Methods ofIsolating Zones in Subterranean formations using Self-Degrading CementCompositions,” and U.S. patent application Ser. No. ______. AttorneyDocket Number HES 2005-IP-016969U2, entitled “Kickoff Plugs Comprising aSelf-Degrading Cement in Subterranean Well Bores,” both filed on thesame day herewith, the entirety of both of which is herein incorporatedby reference.

BACKGROUND

The present invention relates to subterranean drilling operations. Moreparticularly, the present invention relates to methods of drilling awell bore in a subterranean formation comprising placing aself-degrading kickoff plug or the like in the well bore.

During drilling of a well for production of oil, gas, or other fluids,it may be desirable to perform directional drilling, which may result indrilling of a deviated well bore. Directional drilling operations may beconducted for a variety of reasons, including, but not limited to,evading obstructions (e.g., drilling equipment that may have becomestuck in the original well bore), drilling multiple wells from a singlevertical well bore, or increasing production by increasing flow fromadjacent subterranean formations.

Directional drilling operations may involve the setting of a kickoffplug, or the like, in a primary well bore. A kickoff plug may have alength ranging from about 50 to about 500 feet, and may comprise acement composition. The kickoff plug typically is set in the well boreby lowering a drillstring or an open-ended tubing string to the desireddepth and pumping a cement composition into the well bore. The cementcomposition may set to form a plug. After the cement plug has beenformed, a drillstring may be used to reinitiate drilling operations. Thedrillstring and drill bit intentionally may be contacted with the plug,so as to thereby deflect the drill string and change the direction inwhich subsequent drilling proceeds.

The use of conventional kickoff plugs may be problematic, for a varietyof reasons. Conventional kickoff plugs may be difficult to remove fromthe primary well bore, and may block the well bore and/or impairproduction of hydrocarbons from the subterranean formation. Even whereremoval of the kickoff plug from the well bore is possible, the removalprocedure may require additional trips into the well bore, adding costto the drilling operation. Furthermore, in cases in which a mechanicalkickoff plug assembly is used, retrieving the kickoff plug by pulling itback up through the well bore may be problematic, because the well boremay lack sufficient space through which to pull the kickoff plug withoutdamaging upper portions of the well bore and/or casing strings settherein.

Conventional approaches to solving these problems have included, interalia, the use of a salt plug which, once used, can be dissolved by theintroduction of an acidic solution into the well bore. However,significant drawbacks may be associated with this approach, including,but not limited to, environmental and occupational safety risks that mayresult from the use of large quantities of the acidic solution, the riskthat a portion of the acidic solution may escape into other regions ofthe subterranean formation, and the delay of waiting for the acidicsolution to dissolve the plug.

Another conventional approach involves drilling through the kickoffplug. However, this approach may require the use of other drillingequipment (e.g., drilling strings capable of producing a greater force,and a stabilizer assembly to keep the drill string from being deflectedby the plug) that may further complicate the drilling operation and/orrisk damage to the well bore.

SUMMARY

The present invention relates to subterranean drilling operations. Moreparticularly, the present invention relates to methods of drilling awell bore in a subterranean formation comprising placing aself-degrading kickoff plug or the like in the well bore.

An example of a method of the present invention is a method comprising:providing a self-degrading cement composition that comprises adegradable material, an acid source, a base source, and a water source;placing the self-degrading cement composition in a desired location in awell bore that penetrates a subterranean formation; and allowing theself-degrading cement composition to set to form a hardened kickoffplug.

Another example of a method of the present invention is a method ofplacing a degradable kickoff plug at a desired location in a well borepenetrating a subterranean formation, the method comprising: providing aself-degrading cement composition that comprises a degradable material,an acid source, a base source, and a water source; placing theself-degrading cement composition in the desired location in the wellbore; and allowing the self-degrading cement composition to set to formthe degradable kickoff plug.

Another example of a method of the present invention is a method ofdrilling a directional hole through the side of a well bore penetratinga subterranean formation, the method comprising: providing aself-degrading cement composition that comprises a degradable material,an acid source, a base source, and a water source; placing theself-degrading cement composition in a desired location in the wellbore; allowing the self-degrading cement composition to set to form ahardened kickoff plug; contacting the hardened kickoff plug with a drillstring such that the path of the drill string deviates away from thewell bore; and allowing the drill string to continue drilling adirectional hole that deviates from the well bore.

The features and advantages of the present invention will be apparent tothose skilled in the art. While numerous changes may be made by thoseskilled in the art, such changes are within the spirit of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present invention, and should not be used to limit or define theinvention.

FIG. 1 illustrates the relationship of the time- andtemperature-dependence of the degradation of a degradable material inone embodiment of the present invention.

FIG. 2 illustrates the relationship of the time- andtemperature-dependence of the degradation of a degradable material inanother embodiment of the present invention.

FIG. 3 illustrates an example of an oil/gas reservoir in a subterraneanformation.

FIG. 4 illustrates an example of a well bore in the subterraneanformation of FIG. 3.

FIG. 5 illustrates the well bore of FIG. 4, having disposed therein anembodiment of a self-degrading cement composition of the presentinvention.

FIG. 6 illustrates the well bore of FIG. 5, in which the self-degradingcement composition has set to form a set cement.

FIG. 7 illustrates the well bore of FIG. 6, in which a directional holehas been drilled.

FIG. 8 illustrates the well bore of FIG. 7, in which the set cement hascompletely degraded, in accordance with an embodiment of the presentinvention.

FIG. 9 illustrates an example of a well bore, having casing disposedtherein, in the subterranean formation of FIG. 3.

FIG. 10 illustrates the well bore of FIG. 9, having disposed therein anembodiment of a self-degrading cement composition of the presentinvention.

FIG. 11 illustrates the well bore of FIG. 10, in which theself-degrading cement composition has set to form a set cement.

FIG. 12 illustrates the well bore of FIG. 11, in which a directionalhole has been drilled.

FIG. 13 illustrates the well bore of FIG. 12, in which the set cementhas completely degraded, in accordance with an embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to subterranean drilling operations. Moreparticularly, the present invention relates to methods of drilling awell bore in a subterranean formation comprising placing aself-degrading kickoff plug or the like in the well bore.

1. Self-Degrading Cement Compositions

The self-degrading cement compositions utilized in the methods of thepresent invention generally comprise a degradable material, an acidsource, a base source, and a water source. In certain embodiments of thepresent invention, the self-degrading cement composition initially willset to form a hardened mass that provides sufficient structuralintegrity to permit kickoff to occur, after which the degradablematerial may degrade either entirely, or partially. In certainembodiments in which the degradable material degrades only partially,such partial degradation may create voids within the hardened mass ofthe self-degrading cement composition that may permit fluidcommunication through the region of the well bore in which it is placed.

A broad variety of acid sources and base sources may be suitable for usein the self-degrading cement compositions utilized in the methods of thepresent invention. Examples of suitable acid sources include, interalia, magnesium chloride (MgCl₂), potassium phosphate monobasic(KH₂PO₄), phosphoric acid (H₃PO₄), magnesium sulfate (MgSO₄), sodiumphosphate monobasic (NaH₂PO₄), and ammonium phosphate monobasic(NH₆PO₄). Examples of suitable base sources include, inter alia,magnesium oxide (MgO), and ammonia (NH₃). An example of a suitablesource of magnesium oxide is commercially available from Martin Mariettaunder the trade name “MagChem 10.” An example of a suitable source ofpotassium phosphate monobasic is commercially available from FisherScientific.

Generally, an acid source and base source may be chosen that may reactso as to form an acid-base cement. For example, magnesium oxide may bechosen as a base source, and potassium phosphate monobasic may be chosenas an acid source, so that in the presence of water they may react toproduce an acid-base cement having the chemical formula MgKPO₄.6H₂O. Asanother example, magnesium oxide may be chosen as a base source, andmagnesium chloride may be chosen as an acid source, so that in thepresence of water they may react to produce an acid-base cement havingthree oxychloride phases; one oxychloride phase may have the chemicalformula 5Mg(OH)₂MgCl₂.8H₂O, which may be referred to as “5-form.” Asanother example, magnesium oxide may be chosen as a base source, andphosphoric acid may be chosen as an acid source, so that in the presenceof water they may react to produce an acid-base cement having thechemical formula MgHPO₄.3H₂O. As still another example, magnesium oxidemay be chosen as a base source, and magnesium sulfate may be chosen asan acid source, so that in the presence of water they may react toproduce an acid-base cement having four possible oxysulfate phases; oneoxysulfate phase may have the chemical formula 3 Mg(OH)₂MgSO₄.8H₂O,which may be referred to as “3-form.” As still another example,magnesium oxide may be chosen as a base source, and ammonium phosphatemonobasic may be chosen as an acid source, so that in the presence ofwater they may react to produce an acid-base cement having the chemicalformula Mg(NH₄)PO₄.6H₂O. A broad variety of acid sources and basesources may be used, and a broad variety of acid-base cements may beproduced, in accordance with the present invention, including, but notlimited to, those acid sources, base sources, and acid-base cements thatare disclosed in “Acid-Base Cements: Their Biomedical and IndustrialApplications,” by Alan D. Wilson and John W. Nicholson (Cambridge Univ.Press, 1993).

Generally, the acid source and base source may be present in theself-degrading cement composition in a stoichiometric amount. Forexample, in certain embodiments of the present invention whereinmagnesium oxide is used as a base source and potassium phosphatemonobasic is used as an acid source, their relative concentrations maybe illustrated by Equation 1 below:0.15 grams MgO+0.52 grams KH₂PO₄+0.33 grams H₂O→1 gram MgKPO₄.6H₂O  EQUATION 1Equation 1 is exemplary only, and may be modified as one of ordinaryskill in the art will recognize, with the benefit of this disclosure.For example, additional quantities of magnesium oxide may be included,in amounts in the range of from about 1% excess by weight to about 25%excess by weight.

The self-degrading cement compositions utilized in the methods of thepresent invention generally comprise a water source. The water sourcemay comprise fresh water, salt water (e.g., water containing one or moresalts dissolved therein), brine (e.g., saturated salt water), orseawater. Generally, any water source may be used, provided that it doesnot contain an excess of compounds that may adversely affect othercomponents in the self-degrading cement composition.

A broad variety of materials may be suitable as the degradable materialsin the self-degrading cement compositions utilized in the methods of thepresent invention. In certain embodiments of the present invention, thedegradable material may be a degradable polymer. The terms “degradation”or “degradable” refer to both the two relatively extreme cases ofhydrolytic degradation that the degradable material may undergo, e.g.,heterogeneous (or bulk erosion) and homogeneous (or surface erosion),and any stage of degradation in between these two. This degradation canbe a result of, inter alia, a chemical or thermal reaction, or areaction induced by radiation. The terms “polymer” or “polymers” as usedherein do not imply any particular degree of polymerization; forinstance, oligomers are encompassed within this definition.

A polymer is considered to be “degradable” herein if it is capable ofundergoing an irreversible degradation when used in subterraneanapplications, e.g., in a well bore. The term “irreversible” as usedherein means that the degradable material should degrade in situ (e.g.,within a well bore) but should not recrystallize or reconsolidate insitu after degradation (e.g., in a well bore).

The degradability of a degradable polymer often depends, at least inpart, on its backbone structure. For instance, the presence ofhydrolyzable and/or oxidizable linkages in the backbone often yields amaterial that will degrade as described herein. The rates at which suchpolymers degrade are dependent on the type of repetitive unit,composition, sequence, length, molecular geometry, molecular weight,morphology (e.g., crystallinity, size of spherulites, and orientation),hydrophilicity, hydrophobicity, surface area, and additives. Also, theenvironment to which the polymer is subjected may affect how itdegrades, e.g., temperature, presence of moisture, oxygen,microorganisms, enzymes, pH, and the like.

Suitable examples of degradable polymers that may be used in accordancewith the present invention include, but are not limited to, thosedescribed in the publication of Advances in Polymer Science, Vol. 157entitled “Degradable Aliphatic Polyesters,” edited by A. C. Albertsson,pages 1-138. Specific examples include homopolymers, random, block,graft, and star- and hyper-branched aliphatic polyesters. Such suitablepolymers may be prepared by polycondensation reactions, ring-openingpolymerizations, free radical polymerizations, anionic polymerizations,carbocationic polymerizations, coordinative ring-openingpolymerizations, as well as by any other suitable process. Examples ofsuitable degradable polymers that may be used in conjunction with themethods of this invention include, but are not limited to, aliphaticpolyesters; poly(lactides); poly(glycolides); poly(ε-caprolactones);poly(hydroxy ester ethers); poly(hydroxybutyrates); poly(anhydrides);polycarbonates; poly(orthoesters); poly(amino acids); poly(ethyleneoxides); poly(phosphazenes); poly ether esters, polyester amides,polyamides, and copolymers or blends of any of these degradablepolymers, and derivatives of these degradable polymers. The term“copolymer” as used herein is not limited to the combination of twopolymers, but includes any combination of polymers, e.g., terpolymersand the like. As referred to herein, the term “derivative” is definedherein to include any compound that is made from one of the listedcompounds, for example, by replacing one atom in the base compound withanother atom or group of atoms. Of these suitable polymers, aliphaticpolyesters such as poly(lactic acid), poly(anhydrides),poly(orthoesters), and poly(lactide)-co-poly(glycolide) copolymers arepreferred. Poly(lactic acid) is especially preferred. Poly(orthoesters)also may be preferred. Other degradable polymers that are subject tohydrolytic degradation also may be suitable. One's choice may depend onthe particular application and the conditions involved. Other guidelinesto consider include the degradation products that result, the time forrequired for the requisite degree of degradation, and the desired resultof the degradation (e.g., voids).

Aliphatic polyesters degrade chemically, inter alia, by hydrolyticcleavage. Hydrolysis can be catalyzed by either acids or bases.Generally, during the hydrolysis, carboxylic end groups may be formedduring chain scission, which may enhance the rate of further hydrolysis.This mechanism is known in the art as “autocatalysis,” and is thought tomake polyester matrices more bulk-eroding.

Suitable aliphatic polyesters have the general formula of repeatingunits shown below:

where n is an integer between 75 and 10,000 and R is selected from thegroup consisting of hydrogen, alkyl, aryl, alkylaryl, acetyl,heteroatoms, and mixtures thereof. In certain embodiments of the presentinvention wherein an aliphatic polyester is used, the aliphaticpolyester may be poly(lactide). Poly(lactide) is synthesized either fromlactic acid by a condensation reaction or, more commonly, byring-opening polymerization of cyclic lactide monomer. Since both lacticacid and lactide can achieve the same repeating unit, the general termpoly(lactic acid) as used herein refers to writ of formula I without anylimitation as to how the polymer was made (e.g., from lactides, lacticacid, or oligomers), and without reference to the degree ofpolymerization or level of plasticization.

The lactide monomer exists generally in three different forms: twostereoisomers (L- and D-lactide) and racemic D,L-lactide (meso-lactide).The oligomers of lactic acid and the oligomers of lactide are defined bythe formula:

where m is an integer in the range of from greater than or equal toabout 2 to less than or equal to about 75. In certain embodiments, m maybe an integer in the range of from greater than or equal to about 2 toless than or equal to about 10. These limits may correspond to numberaverage molecular weights below about 5,400 and below about 720,respectively. The chirality of the lactide units provides a means toadjust, inter alia, degradation rates, as well as physical andmechanical properties. Poly(L-lactide), for instance, is asemicrystalline polymer with a relatively slow hydrolysis rate. Thiscould be desirable in applications of the present invention in which aslower degradation of the degradable material is desired.Poly(D,L-lactide) may be a more amorphous polymer with a resultantfaster hydrolysis rate. This may be suitable for other applications inwhich a more rapid degradation may be appropriate. The stereoisomers oflactic acid may be used individually, or may be combined in accordancewith the present invention. Additionally, they may be copolymerizedwith, for example, glycolide or other monomers like ε-caprolactone,1,5-dioxepan-2-one, trimethylene carbonate, or other suitable monomersto obtain polymers with different properties or degradation times.Additionally, the lactic acid stereoisomers can be modified by blendinghigh and low molecular weight polylactide or by blending polylactidewith other polyesters. In embodiments wherein polylactide is used as thedegradable material, certain preferred embodiments employ a mixture ofthe D and L stereoisomers, designed so as to provide a desireddegradation time and/or rate. Examples of suitable sources of degradablematerial are poly(lactic acids) that are commercially available fromCargill Dow under the trade names “6250D” and “5639A.”

Aliphatic polyesters useful in the present invention may be prepared bysubstantially any of the conventionally known manufacturing methods suchas those described in U.S. Pat. Nos. 6,323,307; 5,216,050; 4,387,769;3,912,692; and 2,703,316, the relevant disclosures of which areincorporated herein by reference.

Polyanhydrides are another type of degradable polymer that may besuitable for use in the present invention. Polyanhydride hydrolysisproceeds, inter alia, via free carboxylic acid chain-ends to yieldcarboxylic acids as final degradation products. Their erosion time canbe varied over a broad range of changes in the polymer backbone.Examples of suitable polyanhydrides include poly(adipic anhydride),poly(suberic anhydride), poly(sebacic anhydride), and poly(dodecanedioicanhydride). Other suitable examples include, but are not limited to,poly(maleic anhydride) and poly(benzoic anhydride).

The physical properties of degradable polymers may depend on severalfactors including, but not limited to, the composition of the repeatunits, flexibility of the chain, presence of polar groups, molecularmass, degree of branching, crystallinity, and orientation. For example,short chain branches may reduce the degree of crystallinity of polymerswhile long chain branches may lower the melt viscosity and may impart,inter alia, extensional viscosity with tension-stiffening behavior. Theproperties of the material utilized further may be tailored by blending,and copolymerizing it with another polymer, or by a change in themacromolecular architecture (e.g., hyper-branched polymers, star-shaped,or dendrimers, and the like). The properties of any such suitabledegradable polymers (e.g., hydrophobicity, hydrophilicity, rate ofdegradation, and the like) can be tailored by introducing selectfunctional groups along the polymer chains. For example,poly(phenyllactide) will degrade at about one-fifth of the rate ofracemic poly(lactide) at a pH of 7.4 at 55° C. One of ordinary skill inthe art, with the benefit of this disclosure, will be able to determinethe appropriate functional groups to introduce to the polymer chains toachieve the desired physical properties of the degradable polymers.

Whichever degradable material is used in the present invention, thedegradable material may have any shape, including, but not limited to,particles having the physical shape of platelets, shavings, flakes,ribbons, rods, strips, spheroids, toroids, pellets, tablets, or anyother physical shape. In certain embodiments of the present invention,the degradable material used may comprise a mixture of fibers andspherical particles. One of ordinary skill in the art, with the benefitof this disclosure, will recognize the specific degradable material thatmay be used in accordance with the present invention, and the preferredsize and shape for a given application.

In certain embodiments of the present invention, the degradable materialused may comprise a self-degrading fiber that comprises an outer shelland a core liquid, wherein the outer shell comprises a degradablepolymer and substantially retains the core liquid. In certainembodiments of the present invention, the outer shell may comprise adegradable polymer that is subject to hydrolytic degradation. The coreliquid may comprise a liquid that is able to at least partiallyfacilitate or catalyze the hydrolysis of the degradable polymer in theouter shell. Optionally, the self-degrading fiber may comprise a coatingon the outer shell and/or a suitable additive within the core liquid,e.g., an additive chosen to interact with the degradable polymer, itsdegradation products, or the surrounding subterranean environment. Incertain embodiments, the outer shell may be non-porous. Methods ofmaking the self-degrading fibers described herein include any suitablemethod for forming hollow fibers. One such method involves extrudinghollow fibers made from a desired degradable polymer, soaking the hollowfibers in a liquid that will be the core liquid, saturating the hollowfibers with the liquid, and drying the exterior of the outer core of thefibers in such a manner that the liquid is retained in the hollow fibersand becomes a core liquid. Another method involves extruding a spinningsolution of a chosen degradable polymer from an annular slit of a doublepipe orifice to form a sheath solution while simultaneously extruding aliquid through the inside pipe of the double pipe orifice, to form acore liquid within the hollow fibers. Another method involves usingcapillary action to place the core liquid in an already-formed suitablehollow fiber. Other suitable methods known in the art may be used aswell.

In choosing the appropriate degradable material, one should consider thedegradation products that will result, and choose a degradable materialthat will not yield degradation products that would adversely affectother operations or components utilized in that particular application.The choice of degradable material also may depend, at least in part, onthe conditions of the well (e.g., well bore temperature). For instance,lactides have been found to be suitable for lower temperature wells,including those within the range of 60° F. to 150° F., and polylactideshave been found to be suitable for well bore temperatures above thisrange.

In certain embodiments, the degradation of the degradable material couldresult in a final degradation product having the potential to affect thepH of the self-degrading cement compositions utilized in the methods ofthe present invention. For example, in certain embodiments wherein thedegradable material is poly(lactic acid), the degradation of thepoly(lactic acid) to produce lactic acid may alter the pH of theself-degrading cement composition. In certain embodiments, a buffercompound may be included within the self-degrading cement compositionsutilized in the methods of the present invention in an amount sufficientto neutralize the final degradation product. Examples of suitable buffercompounds include, but are not limited to, calcium carbonate, magnesiumoxide, ammonium acetate, and the like. One of ordinary skill in the art,with the benefit of this disclosure, will be able to identify the propertype and concentration of a buffer compound to include in theself-degrading cement composition for a particular application. Anexample of a suitable buffer compound comprises ammonium acetate and iscommercially available from Halliburton Energy Services, Inc., under thetrade name “BA-20.”

The degradable materials utilized in the methods of the presentinvention may degrade over time at a rate that depends upon, among otherthings, the well bore temperature. Referring now to FIGS. 1 and 2,illustrated therein are graphical relationships of the time- andtemperature-dependence of the degradation of certain degradablematerials. The experiment in which these data were obtained wasconducted as follows. A synthetic sea water solution was prepared byadding 41.953 grams of sea salt to one liter of deionized water. Next,1.33 grams of sodium p-toluene sulfonate was added to the sea watersolution to form a solution that was 6.919 mM in sodium p-toluenesulfonate. Next, one gram of a degradable material (6250D or 5639A) wasplaced in a one liter round-bottom flask containing 500 mL of syntheticsea water solution. A reflux condenser then was placed on each flask,and the contents were heated to 75, 85 or 95° C.

Using a disposable pipette, an aliquot was removed from each flask andplaced in a 10 mL beaker. A carefully measured aliquot of 5.00 mL wasremoved and placed in a 50 mL round-bottom flask. The contents of theflasks were frozen by placing the flasks in liquid nitrogen. The flasksthen were placed on a high vacuum line and the samples were allowed todry overnight. After 24 hours, 1 mL of D₂O was added to each flask, andthe contents of the flask were stirred until the residue re-dissolved.The freeze drying was repeated to remove D₂O and residual water. Theremaining materials were dissolved in D₂O for NMR measurement.

The ¹H NMR spectrum was collected using a Bruker 300 Avance NMRspectrometer operating at 300 MHz, using a 5 mm QNP probe at varioustime intervals. The integrated area of the methyl proton peak of lacticacid was compared to the integrated area of the 6.919 mM sodiump-toluene sulfonate internal standard, and the lactic acid concentrationfor each point displayed in FIGS. 1 and 2 was calculated from thatratio. FIG. 1 illustrates the time- and temperature-dependence of thegeneration of lactic acid caused by the degradation of 6250D, while FIG.2 illustrates the time- and temperature-dependence of the generation oflactic acid caused by the degradation of 5639A.

For certain embodiments of the self-degrading cement compositionsutilized in the methods of the present invention wherein poly(lacticacid) is used as the degradable material, Table 1 below demonstrates therelationship that may exist between the concentration of poly(lacticacid) in the self-degrading cement composition and the degree of voidspace that may result in the solid mass after the poly(lactic acid) isallowed to degrade. TABLE 1 Poly(lactic acid) concentration (volumepercent of the cement composition) Resulting void space 8% 20% 11% 30%13% 40% 15% 50%

Optionally, the self-degrading cement compositions utilized in themethods of the present invention may include a set retarder. Generally,any set retarder may be used with the self-degrading cement compositionsutilized in the methods of the present invention. Examples of setretarders suitable for use in the self-degrading cement compositionsutilized in the methods of the present invention include, but are notlimited to, sodium citrate and sodium borate. An example of a suitablecommercially-available set retarder is Component R, available fromHalliburton Energy Services, Inc., of Duncan, Okla. Where included, theset retarder may be present in the self-degrading cement compositionsutilized in the methods of the present invention in an amount in therange of from about 0.05% to about 10% by weight of the self-degradingcement composition. In certain embodiments, the set retarder may bepresent in the self-degrading cement compositions utilized in themethods of the present invention in an amount in the range of from about0.1% to about 4% by weight of the self-degrading cement composition.

The self-degrading cement compositions utilized in the methods of thepresent invention optionally may include a strength-enhancing additive,which may act, among other things, to increase the stability of the setcement. Examples of these strength-enhancing additives include, but arenot limited to, Newberyite, calcium carbonate, and Struvite. Whereincluded, the strength-enhancing additive may be present in theself-degrading cement compositions utilized in the methods of thepresent invention in an amount in the range of from about 5% to about60% by weight of the self-degrading cement composition. In certainembodiments, the strength-enhancing additive may be present in theself-degrading cement compositions utilized in the methods of thepresent invention in an amount in the range of from about 10% to about30% by weight of the self-degrading cement composition.

Examples of other additional additives that may be added to theself-degrading cement compositions of the present invention include,among other things, fluid loss control additives, salts, vitrifiedshale, fly ash, fumed silica, bentonite, viscosifiers, suspendingagents, dispersants, and the like. An example of a suitable fly ash is“POZMIX® A,” commercially available from Halliburton Energy Services,Inc., of Duncan, Okla. An example of a suitable source of fumed silicais “SILICALITE™,” commercially available from Halliburton EnergyServices, Inc., of Duncan, Okla. An example of a suitable viscosifier is“VERSASET™,” commercially available from Halliburton Energy Services,Inc., of Duncan, Okla. One skilled in the art, with the benefit of thisdisclosure, will be able to determine which additional additives areappropriate for a particular application of the methods of the presentinvention, as well as the amounts of those additives that should beused.

2. Methods of Forming and Using Kickoff Plugs

In one embodiment, the present invention provides a method of placing aplug in a well bore in a subterranean formation. The well bore in whichthe self-degrading cement composition is placed may be an open hole, acased hole, or any combination thereof.

Referring now to FIG. 3, a cross-section of a subterranean formation isillustrated therein. In the formation illustrated in FIG. 3, a reservoir1 (comprising hydrocarbons, e.g., oil and/or gas) is depicted, boundedabove and below by shale zones. Reservoir 1 has an upper portion 30 anda lower portion 35.

Referring now to FIG. 4, a well bore 100 has been drilled within theformation. As shown in FIG. 4, production of hydrocarbons has beencommenced, which gradually will cause upper portion 30 of reservoir 1 tobe depleted over time; hydrocarbons will remain in lower portion 35 ofreservoir 1.

Referring now to FIG. 5, a self-degrading cement composition 2 is placedin well bore 100, via any suitable manner. In some embodiments, theself-degrading cement composition 2 may be pumped through thedrillstring. In some embodiments, the self-degrading cement composition2 may be pumped through an open-ended coiled tubing 160 to the desiredlocation in well bore 100. One skilled in the art, with the benefit ofthis disclosure, will recognize the appropriate techniques and equipmentfor placing the self-degrading cement composition 2 in well bore 100 ina particular application. In some embodiments, the self-degrading cementcomposition 2 may be placed in well bore 100 just below the point atwhich a directional hole or well bore will be initiated.

After the placement of the self-degrading cement composition 2 withinthe subterranean formation, the water source within the self-degradingcement composition 2 may combine with the dry materials in theself-degrading cement composition 2 to form what may be referred to as a“hydrate,” e.g., a solid compound comprising water molecules that maycombine in a definite ratio. Furthermore, the water molecules within thehydrate may provide a hydrolysis source for the degradable material.

The amount of time required for the self-degrading cement composition 2to set to form a hardened kickoff plug 150 may depend upon a variety offactors, including, but not limited to, the temperature in well bore100, the desired size and/or strength of the kickoff plug 150, theformulation of the self-degrading cement composition 2, and/or thepresence of a set retarder. One of ordinary skill in the art, with thebenefit of this disclosure, will recognize the amount of time requiredfor the self-degrading cement composition 2 to set. In some embodimentsof the present invention, permitting the self-degrading cementcomposition 2 to set to form a hardened kickoff plug 150 may requirewaiting an amount of time in the range of from about 15 minutes to about72 hours.

Referring now to FIG. 6, the self-degrading cement composition placedwithin well bore 100 has set to form kickoff plug 150.

In some embodiments, the methods of the present invention further maycomprise contacting the hardened kickoff plug 150 with a drill stringsuch that the path of the drill string deviates away from well bore 100to a desired degree and allows the drill string to continue drilling adirectional hole at a desired deviation from well bore 100. Referringnow to FIG. 7, a directional hole 250 is shown, which permits fluid flowfrom lower reservoir 35 to well bore 100, and subsequently to thesurface. In certain embodiments, the directional hole 250 may deviateaway from well bore 100 by an amount in the range of from about 1 degreeto about 20 degrees. The desired degree of deviation will depend upon avariety of factors, including, but not limited to, the location ofrecoverable fluids in the subterranean formation or an adjacentformation, the stability of the subterranean formation (e.g., themechanical properties of the formation), the location of an obstructionaround which the directional hole is constructed, and the like. One ofordinary skill in the art, with the benefit of this disclosure, willrecognize the desired degree of deviation from well bore 100 fordirectional hole 250, and will be able to adapt the methods of thepresent invention to drill directional hole 250 at that desireddeviation.

Referring now to FIG. 8, hydrocarbon production has commenced from thelower reservoir 35. In some embodiments of the present invention,including those illustrated in FIG. 8, the hardened kickoff plug 150 maybe allowed to degrade such that fluid communication through the portionof well bore 100 wherein kickoff plug 150 resides is at least partiallyrestored; in preferred embodiments, kickoff plug 150 may degrade to suchextent that fluid communication is completely restored. In someembodiments of the present invention, it may be desirable to allow thedegradable material to degrade slowly over time, rather thaninstantaneously. In certain embodiments, allowing kickoff plug 150 todegrade such that fluid communication through the portion of well bore100 wherein kickoff plug 150 resides is at least partially restored mayrequire waiting an amount of time in the range of from about 4 hours toabout 21 days. In certain embodiments, allowing kickoff plug 150 todegrade such that fluid communication through the portion of well bore100 wherein kickoff plug 150 resides is at least partially restored mayrequire waiting an amount of time in the range of from about 4 hours toabout 36 hours. As illustrated in FIG. 8, enhanced hydrocarbonproduction is depicted, facilitated by the degradation of kickoff plug150 and the drilling of directional hole 250.

Certain embodiments of the methods of the present invention may be usedin well bores comprising casing. Referring now to FIG. 9, across-section of a subterranean formation is illustrated therein. In theformation illustrated in FIG. 9, a reservoir 1 (comprising hydrocarbons,e.g., oil and/or gas) is depicted, bounded above and below by shalezones. Reservoir 1 has an upper portion 30 and a lower portion 35. Wellbore 100 has been drilled within the formation. Casing 120 is disposedwithin well bore 100. A conventional cement composition 109 has beenplaced in an annulus between the outer surface of casing 120 and theinner walls of well bore 100.

Referring now to FIG. 10, a self-degrading cement composition 2 isplaced in well bore 100, via any suitable manner (e.g., by pumpingthrough a drillstring, or, as shown in FIG. 10, by pumping throughopen-ended coiled tubing 160, or the like), as previously has beendescribed herein.

Referring now to FIG. 11, the self-degrading cement composition 2 hasset to form kickoff plug 150.

Referring now to FIG. 12, illustrated therein is a directional hole 250that permits fluid flow from lower reservoir 35 to well bore 100, andsubsequently to the surface. Directional hole 250 may be provided, forexample, by contacting the hardened kickoff plug 150 with a drill string(not shown in FIG. 12) such that the path of the drill string deviatesaway from well bore 100 to a desired degree, thereby permitting thedrill string to continue drilling directional hole 250 at a desireddeviation from well bore 100. The degree to which directional hole 250may deviate away from well bore 100 previously has been describedherein. In certain embodiments, once directional hole 250 has beendrilled to a desired extent, a casing string 320 (not shown in FIG. 12)may be placed within directional hole 250, and perforations 330 (notshown in FIG. 12) may be created within casing string 320 (e.g., by useof a perforating tool, and the like). Subsequently, hydrocarbons (e.g.,in the lower reservoir 35) may flow through perforations 330 (not shownin FIG. 12), into directional hole 250, and flow therefrom to thesurface.

Referring now to FIG. 13, enhanced hydrocarbon production is depicted,facilitated by the drilling of directional hole 250 and the degradationof kickoff plug 150. In some embodiments of the present invention,including those illustrated in FIG. 13, the hardened kickoff plug 150may be allowed to degrade such that fluid communication through theportion of well bore 100 wherein kickoff plug 150 resides is at leastpartially restored; in preferred embodiments, kickoff plug 150 maydegrade to such extent that fluid communication is completely restored.As illustrated in FIG. 13, the degradation of kickoff plug 150facilitates entry of hydrocarbons into well bore 100 throughperforations 290. In some embodiments of the present invention, it maybe desirable to allow the degradable material to degrade slowly overtime, rather than instantaneously.

To facilitate a better understanding of the present invention, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit, or to define, thescope of the invention.

EXAMPLE 1

Sample compositions were formed as follows. First, 7.58 grams ofmagnesium oxide were dry blended with 25.75 grams of potassium phosphatemonobasic crystals (KH₂PO₄), and mixed with 16.67 grams of tap water.The mixture was stirred for some time, and poly(lactic acid) (“6250D”)was added, generally in an amount in the range of from about 35% byweight to about 40% by weight. Certain of the sample compositionsfurther comprised an acid-base cement referred to as Newberyite, andhaving the chemical formula MgH(PO₄).3H₂O. Among other things,Newberyite is thought to impart strength-enhancing properties to thesample composition, and the additional water that Newberyite may supplymay facilitate hydrolysis of the degradable material (6250D, in thisexample). Table 2 sets forth the respective amounts of 6250D andNewberyite included in a particular sample composition. TABLE 2 SamplePoly(lactic acid) Composition (“6250D”) Newberyite 1 20 grams Not added2 20 grams Not added 3 20 grams 10 grams 4 15 grams 10 grams 5 15 gramsNot added 6 20 grams 10 grams 7 20 grams Not added 8 20 grams 10 grams

Each sample composition was placed in a 20 mL plastic cylinder, and wasallowed to set therein into a hard rod. Each rod then was left for adesignated cure time at room temperature. Next, the set rod was takenout of the cylinder and either tested for compressibility or directlyplaced in a bomb supplied by PARR Instrument Company, Moline, Ill. Amongother things, the bomb prevented the escape of water that may have beenpresent in the set rod. The bomb was heated in a stove at 250° F. Aftera time (listed as “PARR Time” in Table 3 below), the bomb was removedfrom the stove, and its contents were observed to see whether or notdegradation occurred.

Certain sample compositions were tested for compressibility using anapparatus supplied by Tinius Olsen Company of Willow Grove, Pa. Theprocedure was performed as follows. After the sample composition hadcured and set into a hard rod, the rod was cut down to a 1 inch diameterand a 3 inch length. Two faces of the rod were smoothed. The rod thenwas placed under the Tinius Olsen compressibility load cell andsubjected to a displacement load at a rate of 0.07 inches per minute.The maximum loading that each rod could withstand until failure wasrecorded.

The results of the testing are set forth in Table 3 below. TABLE 3 RodSample Cure Compressive PARR Compo- Time Strength Time sition (75° F.)(psi) (250° F.) Degradation Comments 1 24 — 24 hours Flowable liquidwith hours particulates about 1 mm in diameter. 2 24  290 72 hoursChunks (5-10 mm in hours diameter) with some liquid. 3 24 1560 24 hoursSmall chunks (1-3 mm hours with some liquid); very “sandy.” 4 24 2040 24hours No self-degradation days observed 5 24  510 48 hours Noself-degradation days observed 6 44 2470 (High) 72 hours Noself-degradation hours  490 (Low) observed 7 24  630 24 hours Noself-degradation hours @ 180° F. observed 24 hours Large chunks (>1 cmin @ 250° F. diameter) with some liquid. 8 24 1180 24 hours Noself-degradation hours @ 180° F. observed 24 hours Large chunks (>1 cmin @ 250° F. diameter) with some liquid.

Example 1 demonstrates, inter alia, that the combination of a degradablematerial and an acid-base cement may be suitable for use in the methodsof the present invention.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Whilenumerous changes may be made by those skilled in the art, such changesare encompassed within the spirit of this invention as defined by theappended claims. The terms in the claims have their plain, ordinarymeaning unless otherwise explicitly and clearly defined by the patentee.

1. A method comprising: providing a self-degrading cement compositionthat comprises a degradable material, an acid source, a base source, anda water source; placing the self-degrading cement composition in adesired location in a well bore that penetrates a subterraneanformation; and allowing the self-degrading cement composition to set toform a hardened kickoff plug.
 2. The method of claim 1 wherein thedegradable material comprises at least one of the following group: analiphatic polyester; a polysaccharide; a poly(lactide); apoly(glycolide); a poly(ÿ-caprolactone); a protein; apoly(hydroxybutyrate); a poly(anhydride); an aliphatic polycarbonate; anortho ester; a poly(orthoester); a poly(vinylacetate); a poly(hydroxyester ether); a poly(amino acid); a poly(ethylene oxide); chitin;chitosan; a polyphosphazene; a poly ether ester; a polyester amide; apolyamide; and a derivative thereof.
 3. The method of claim 1 whereinthe degradable material comprises poly(lactic) acid.
 4. The method ofclaim 1 wherein the degradable material comprises a self-degrading fiberthat comprises an outer shell and a core liquid, wherein the outer shellcomprises a degradable polymer and substantially retains the coreliquid.
 5. The method of claim 4 wherein the self-degrading fibercomprises a coating on the outer shell.
 6. The method of claim 4 whereinthe self-degrading fiber comprises an additional additive within thecore liquid.
 7. The method of claim 1 wherein the self-degrading cementcomposition further comprises a buffer compound.
 8. The method of claim1 wherein the acid source comprises at least one of the following group:magnesium chloride; potassium phosphate monobasic; phosphoric acid;magnesium sulfate; ammonium phosphate monobasic; and a derivativethereof.
 9. The method of claim 1 wherein the base source comprises atleast one of the following group: magnesium oxide; ammonia; and aderivative thereof.
 10. The method of claim 1 wherein the self-degradingcement composition further comprises a strength-enhancing additive. 11.The method of claim 1 wherein the self-degrading cement compositionfurther comprises a set retarder.
 12. The method of claim 1 furthercomprising contacting the hardened kickoff plug with a drill string soas to cause the path of the drill string to deviate away from the wellbore.
 13. The method of claim 12 further comprising allowing the drillstring to continue drilling a directional hole that deviates from thewell bore.
 14. The method of claim 1 further comprising allowing thehardened kickoff plug to degrade such that fluid communication throughthe portion of well bore where the kickoff plug was disposed is at leastpartially restored.
 15. The method of claim 13 further comprisingallowing the hardened kickoff plug to degrade such that fluidcommunication through the portion of well bore where the kickoff plugwas disposed is at least partially restored.
 16. A method of placing adegradable kickoff plug at a desired location in a well bore penetratinga subterranean formation, the method comprising: providing aself-degrading cement composition that comprises a degradable material,an acid source, a base source, and a water source; placing theself-degrading cement composition in the desired location in the wellbore; and allowing the self-degrading cement composition to set to formthe degradable kickoff plug.
 17. The method of claim 16 wherein thedesired location in the well bore is located just below the point in thewell bore at which a directional hole through the side of the well boreis to be drilled.
 18. A method of drilling a directional hole throughthe side of a well bore penetrating a subterranean formation, the methodcomprising: providing a self-degrading cement composition that comprisesa degradable material, an acid source, a base source, and a watersource; placing the self-degrading cement composition in a desiredlocation in the well bore; allowing the self-degrading cementcomposition to set to form a hardened kickoff plug; contacting thehardened kickoff plug with a drill string such that the path of thedrill string deviates away from the well bore; and allowing the drillstring to continue drilling a directional hole that deviates from thewell bore.
 19. The method of claim 18 wherein allowing the drill stringto continue drilling a directional hole comprises drilling through atleast a portion of a casing string in the well bore.
 20. The method ofclaim 18 further comprising allowing the hardened kickoff plug todegrade such that fluid communication through the portion of well borewhere the kickoff plug resides is at least partially restored.